If you are using a USB thumbdrive, it may be interesting to be able to mount it as a normal user, i.e. without the need for a privileged account. The question is how to set these rights for devices not available at boot time?... Just use devfs.rules(5), it serves the same purpose than /etc/devfs.conf, but is specifically well suited for removable devices.

Here is an example of setting the mode and group (in which must resides the desired user(s)) for a USB removable key, seen as an SCSI device of the direct access class:

$ cat /etc/devfs.rules
[devfsrules_allow_usb=10]
add path 'da*' mode 0660 group usb

# Devices usually not available at boot time.
[devfsrules_allow=100]
add include $devfsrules_allow_usb

Many thanks to Roland Smith for its particular interesting notes on that subject and for contributing the devfs.conf(5) and devfs.rules(5) manual pages to the FreeBSD project.